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Old 25th June 2011, 02:17 PM   #1
Join Date: Jun 2011
Default 200W SMPS for Class D guitar amp


I wish to build an SMPS to power a 200W class D audio amp for electric guitar.

As you know, most of the time the SMPS would not be supplying 200W.

-the average power drawn from this SMPS, as you know, will be around 30W.

Therefore, is it normal to simply build such an SMPS, by building an SMPS with a maximum power output of , say 70W, and just have a large output capacitor bank.

....when the Amp is running on high power (200W), then the supply rails will simply dip down a bit....which is not such a problem , since it may even enhance the guitar sound.

So, is this correct, can i simply use a 70W power limited SMPS for the 200W class D audio amplifier?
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Old 25th June 2011, 04:31 PM   #2
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Use smps designed for 200w max. It will last longer and run cooler.
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Old 25th June 2011, 05:10 PM   #3
luka is offline luka  Slovenia
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you don't even need large output capacitor bank, depends on voltage

So, is this correct, can i simply use a 70W power limited SMPS for the 200W class D audio amplifier?
indeed you can, if you have enough stored energy
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Old 25th June 2011, 07:11 PM   #4
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Actually, I think you have it exactly backwards, better to have a 70 watt amp on a 200 watt supply, especially with a guitar amp where you typically will be drawing high current much of the time if driven hard. You might get away with it if you don't push it very hard, but larger power supply caps are no substitute for proper design.

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Old 25th June 2011, 08:54 PM   #5
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Class d amps dont clip nicely for a guitar so you need to be supplying the right amount of power.
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Old 26th June 2011, 07:20 PM   #6
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Do you know where i can get a plot of instantaneous power draw for a Class D guitar amp of known Rated Wattage?

......So that i can get an idea of the relationship of peak power to average power, and see how long the peak power draw intervals last.
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Old 26th June 2011, 08:47 PM   #7
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^ Why would you need this? As people have mentioned you should use a 200W minimum, switching PSU. For good lifespan, make it closer to 300W.

However, if it uses an IC for the amp stage, look up the IC's datasheet for output power at the voltage you measure on the power rails under load, play a continuous tone if you don't have a o-scope so you have to measure voltage with a multimeter, but don't play it long, in case the amp can't cope with that continual output power. Once you have that voltage, the datasheet would show power output at that voltage.

Otherwise the spec sheet for the amp might tell you, or if all else fails assume it is 200W.

With a SMPS you do not want a lot of capacitance, it solves no problem as the SMPS would just shut off if you tried to provide too much capacitance (enough to do what you're hoping it would) because the PSU sees it as (close enough to) a short on the output because it can't bring the power rail up to the regulated value fast enough. There are subcircuits you can add to make it work but it's going the wrong direction to make something which wouldn't work, work a little, instead of making it work right in the first place.

If your concern for use of a higher wattage PSU is the cost, use a linear PSU instead as you can throw one together yourself at lower cost than a custom built commercial SMPS unless what you need happens to be the same voltage as lots of consumer products use so there is a competitive market and volume that drives down prices per unit.
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Old 26th June 2011, 09:02 PM   #8
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Guitar amp will squash flat against both rails, power supply needs to be the full wattage.
Sorry, no room for a cheat here. And Class D output clips just awful, with no squashing.
You need to limit before the input with diodes that squash the signal just shy of clipping.
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